How Marginal ROAS Works
It evaluates the slope of the spend-response curve rather than averaging all historical revenue over all spend.
A Simple Example
A campaign may report 5× average ROAS, but the next $10,000 of spend may generate only $25,000 in added revenue—a 2.5× marginal ROAS.
Why Marginal ROAS Matters
Budget decisions concern the return on the next dollar, so marginal ROAS is often more useful for scaling than average ROAS.
Common Misreading
Average ROAS can stay strong while incremental spend is already unprofitable. Do not assume the historical average applies to the next tranche.